Heating resistor flow rate measuring instrument

ABSTRACT

A heating resistor flow rate measuring instrument includes a sub-passage ( 3 ) disposed in a main passage ( 4 ). A heating resistor ( 1 ) and a thermally sensitive resistor ( 2 ) are disposed in the sub-passage ( 3 ). The sub-passage ( 3 ) comprises a first sub-passage having an upstream-side opening face substantially perpendicular to the forward direction of a fluid flow and a downstream-side opening face substantially parallel to the direction opposite to the forward fluid flow, and a second sub-passage having an upstream-side opening face substantially parallel to the forward direction of the fluid flow and a downstream-side opening face substantially perpendicular to the direction opposite to the forward fluid flow. The heating resistor ( 1 ) is disposed in the first sub-passage, and the thermally sensitive resistor ( 2 ) is disposed in the second sub-passage. The heating resistor ( 1 ) is heated to a temperature about 100° C.-300° C. higher than a fluid temperature Ta. The thermally sensitive resistor ( 2 ) is heated higher than the fluid temperature Ta by about {fraction (1/10)} to ½ of the temperature to which the heating resistor ( 1 ) is heated. When the fluid flows in the forward direction, the heating resistor ( 1 ) outputs a value depending on the flow rate of the fluid. When the fluid flows in the opposite direction, the thermally sensitive resistor ( 2 ) is cooled to a temperature lower than that resulting when the fluid flows in the forward direction, and an electric current flowing through the heating resistor ( 1 ) is decreased. As a result, even when a pulsation occurs in the fluid, a precise flow rate can be measured because not only a plus effect caused by a backward flow is eliminated, but also a value corresponding to the backward flow is subtracted.

TECHNICAL FIELD

[0001] The present invention relates to a heating resistor flow ratemeasuring instrument for measuring the flow rate of a fluid when thefluid flowing in the forward direction may cause a pulsationaccompanying a backward flow, and more particularly to a heatingresistor flow rate measuring instrument suitable for measuring the flowrate of intake air of an automobile engine.

BACKGROUND ART

[0002] JP,B 4-34686, for example, discloses one of prior-art heatingresistor flow rate measuring instruments.

[0003] Also, JP,A 8-159833, for example, discloses one of heatingresistor flow rate measuring instruments capable of measuring the flowrate of a fluid when the fluid flowing in the forward direction maycause a pulsation accompanying a backward flow.

[0004] In the flow rate measuring instrument disclosed in JP,A 8-159833,a sub-passage is provided in a main passage, and a heating resistor isdisposed in the sub-passage. The sub-passage includes a pair ofsub-passage portions that are extended substantially parallel to acenter axis of the main passage and are opened in directions opposite toeach other. An end of one sub-passage portion positioned away from theopening side has a communicating portion communicated with the vicinityof an opening of the other sub-passage portion. The heating resistor isdisposed in each sub-passage between the two communication portions.

DISCLOSURE OF INVENTION

[0005] In the prior-art flow rate measuring instrument disclosed in theabove-cited JP,B 4-34686, however, when a backward pulsating flow occursin the fluid flowing in the forward direction, it is difficult tocompletely eliminate the effect of the backward pulsating flow and toprecisely measure the flow rate of the forward flow.

[0006] Also, in the flow rate measuring instrument disclosed in theabove-cited JP,A 8-159833, the effect of the backward pulsating flow canbe eliminated to some extent, but a plurality of heating resistors andat least one thermally sensitive resistor are required, thus resultingin a problem that the circuit configuration is complicated.

[0007] The present invention has been made in view of theabove-mentioned problems in the prior art, and its object is to providea heating resistor flow rate measuring instrument which has a simplifiedcircuit configuration and is able to precisely detect the flow rate inthe forward direction even for a fluid that may cause a pulsationaccompanying a backward flow, such as intake air of an automobileengine.

[0008] Particularly, an object of the present invention is to provide anair flow rate measuring instrument in which, in measurement of the flowrate of intake air of an automobile engine, a large plus error resultingwhen a pulsation accompanying a backward flow occurs near a throttlefully-opened stroke in a specific range of revolution speed can beeliminated and fuel control, etc. precisely responsive to operationconditions can be achieved.

[0009] To achieve the above objects, the present invention isconstituted as follows.

[0010] (1) A heating resistor flow rate measuring instrument comprisinga heating resistor and a thermally sensitive resistor both disposed in amain passage, and measuring the flow rate of a fluid passing through themain passage, the instrument including a first location exposed to thefluid flowing in one direction within the main passage in a largeramount than the fluid flowing in a direction opposite to the onedirection; and a second location exposed to the fluid flowing in theopposite direction within the main passage in a larger amount than thefluid flowing in the one direction, wherein the heating resistor isdisposed at the first location, the thermally sensitive resistor isdisposed at the second location, and the flow rate of the fluid passingthrough the main passage is measured based on amounts of heat radiatedfrom the heating resistor and the thermally sensitive resistor.

[0011] (2) In above (1), preferably, the heating resistor is heated tobe higher than a fluid temperature in the main passage by a firstpredetermined temperature, and the thermally sensitive resistor isheated to be higher than the fluid temperature in the main passage by asecond predetermined temperature.

[0012] (3) In above (1) or (2), preferably, the first location isprovided by a first sub-passage having a first opening that facessubstantially perpendicular to the fluid flowing in the one direction,and a second opening that faces substantially parallel to the fluidflowing in the opposite direction, and the second location is providedby a second sub-passage having a third opening that faces substantiallyperpendicular to the fluid flowing in the opposite direction, and athird opening that faces substantially parallel to the fluid flowing inthe one direction.

[0013] (4) In above (1) or (2), preferably, the first location isprovided by a first sub-passage having a first opening that facessubstantially perpendicular to the fluid flowing in the one direction,and a second opening that faces substantially parallel to the fluidflowing in the opposite direction, and a wall portion having a surfacesubstantially perpendicular to the lengthwise direction of the mainpassage is formed in the one-direction side of the second location.

[0014] (5) In above (1) or (2), preferably, the first location isprovided by a first sub-passage having a first opening that facessubstantially perpendicular to the fluid flowing in the one direction,and a second opening that faces substantially perpendicular to the fluidflowing in the opposite direction and has a smaller opening area thanthe first opening, and the second location is provided by a secondsub-passage having a third opening that faces substantiallyperpendicular to the fluid flowing in the one direction, and a fourthopening that faces substantially perpendicular to the fluid flowing inthe opposite direction and has a larger opening area than the thirdopening.

[0015] (6) In above (1), (2), (3), (4) and (5), preferably, thethermally sensitive resistor is heated to a temperature 20° C.-40° C.higher than the fluid temperature in the main passage.

[0016] (7) A heating resistor flow rate measuring instrument formeasuring the flow rate of a fluid passing through a passage, theinstrument comprising a first heating resistor radiating a larger amountof heat to the fluid flowing in one direction within the passage than tothe fluid flowing in a direction opposite to the one direction; and asecond heating resistor radiating a larger amount of heat to the fluidflowing in the opposite direction than to the fluid flowing in the onedirection; wherein a bridge circuit including the first heating resistorand the second heating resistor is formed, the first heating resistor isheated to be higher than the second heating resistor by a certaintemperature, and the flow rate of the fluid passing through the passageis measured based on the amount of heat radiated from the first heatingresistor.

[0017] (8) A heating resistor flow rate measuring instrument comprisinga heating resistor and a thermally sensitive resistor both disposed in amain passage, and measuring the flow rate of a fluid passing through themain passage, wherein the instrument includes athermally-sensitive-resistor arrangement location in which the thermallysensitive resistor is disposed and radiates a larger amount of heat whenthe fluid flows in a direction opposite to one direction within the mainpassage than when the fluid flows in the one direction; the heatingresistor is heated to be higher than a fluid temperature in the passageby a first predetermined temperature; the thermally sensitive resistoris heated to be higher than the fluid temperature in the passage by asecond predetermined temperature; and the flow rate of the fluid passingthrough the passage is measured based on amounts of heat radiated fromthe heating resistor and the thermally sensitive resistor.

[0018] (9) In above (8), preferably, the heating resistor is disposed ata location in which the heating resistor radiates a larger amount ofheat when the fluid flows in the one direction within the main passagethan when the fluid flows in the opposite direction.

[0019] With the present invention, since the passage is constructed suchthat the thermally sensitive resistor is more easily exposed to abackward flow and the heating resistor is harder to be exposed to thebackward flow, the flow rate in the forward direction can be preciselydetected even for a fluid that may cause a pulsation accompanying thebackward flow, such as intake air of an automobile engine.

[0020] Further, the number of resistors used is small and hence theinstrument can be realized with a simple circuit configuration.

[0021] Particularly, in measurement of the flow rate of intake air of anautomobile engine, a large plus error resulting when a pulsationaccompanying a backward flow occurs near a throttle fully-opened strokein a specific range of revolution speed can be eliminated and fuelcontrol precisely responsive to operation conditions can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a diagram showing the basic principle of a heatingresistor flow rate measuring instrument of the present invention.

[0023]FIG. 2 is a graph showing the relationship between an air pressurein an intake manifold, which is a pressure upstream of an enginecylinder, and an output of an air flow rate measuring instrumentresulting when the engine revolution speed is held constant.

[0024]FIG. 3 is a graph showing the relationship between a plus error,which is caused in a prior-art heating resistor flow rate measuringinstrument upon the generation of a pulsation accompanying a backwardflow near a throttle fully-opened stroke, and the flow rate detected bythe flow rate measuring instrument.

[0025]FIG. 4 is a graph showing an output of a bypassing flow ratemeasuring instrument resulting when a pulsation accompanying a backwardflow occurs.

[0026]FIG. 5 is a graph showing the relationship between a plus errorcaused upon the generation of a pulsation accompanying a backward flownear a throttle fully-opened stroke and an output of the flow ratemeasuring instrument shown in FIG. 4.

[0027]FIG. 6 is a graph showing the relationship between a plus error,which is caused in the heating resistor flow rate measuring instrumentof the present invention upon the generation of a pulsation accompanyinga backward flow near a throttle fully-opened stroke, and an output ofthe flow rate measuring instrument.

[0028]FIG. 7 is a sectional view of a heating resistor flow ratemeasuring instrument according to a first embodiment of the presentinvention.

[0029]FIG. 8 is a sectional view taken along the line A-A in FIG. 7.

[0030]FIG. 9 is a sectional view taken along the line B-B in FIG. 7.

[0031]FIG. 10 is a sectional view of a sub-passage in a heating resistorflow rate measuring instrument according to a second embodiment of thepresent invention.

[0032]FIG. 11 is a sectional view of a sub-passage in a heating resistorflow rate measuring instrument according to a third embodiment of thepresent invention.

[0033]FIG. 12 is a schematic sectional view of a heating resistor flowrate measuring instrument according to a fourth embodiment of thepresent invention.

[0034]FIG. 13 is an external appearance view, looking from the upstreamside, of the heating resistor flow rate measuring instrument accordingto the fourth embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0035] Practical embodiments of a heating resistor flow rate measuringinstrument of the present invention will be described below withreference to the drawings.

[0036]FIG. 1 is a diagram showing the basic principle of a heatingresistor flow rate measuring instrument of the present invention. InFIG. 1, it is assumed that a fluid flow directing rightward from theleft side is a forward flow 11, and a fluid flow directing leftward fromthe right side is a backward flow 12.

[0037] The heating resistor flow rate measuring instrument of thepresent invention includes a sub-passage 3 disposed in a main passage 4.A heating resistor 1 (Rh) and a thermally sensitive resistor 2 (Rc) aredisposed in the sub-passage 3.

[0038] The sub-passage 3 comprises a first sub-passage having anupstream-side opening face substantially perpendicular to the forwarddirection of the fluid flow and a downstream-side opening facesubstantially parallel to the direction opposite to the forward fluidflow, and a second sub-passage having an upstream-side opening facesubstantially parallel to the forward direction of the fluid flow and adownstream-side opening face substantially perpendicular to thedirection opposite to the forward fluid flow.

[0039] The heating resistor 1 is disposed in the first sub-passage, andthe thermally sensitive resistor 2 is disposed in the secondsub-passage.

[0040] Thus, the first sub-passage including the heating resistor 1disposed therein is constructed such that the fluid flowing in thebackward direction is harder to flow into the first sub-passage than thefluid flowing in the forward direction. The second sub-passage includingthe thermally sensitive resistor 2 disposed therein is constructed suchthat the fluid flowing in the forward direction is harder to flow intothe second sub-passage than the fluid flowing in the backward direction.

[0041] The heating resistor 1 is heated to a temperature about 100°C.-300° C. higher than a fluid temperature Ta in the first sub-passage,and provides an output depending on the flow rate of the fluid inaccordance with the amount of heat radiated to the fluid.

[0042] Although the thermally sensitive resistor 2 is used in the priorart to detect the fluid temperature without being heated, the thermallysensitive resistor 2 in the present invention is heated to be higherthan the fluid temperature Ta by about {fraction (1/10)} to ½ of thetemperature to which the heating resistor 1 is heated.

[0043] For example, the heating resistor 1 is heated to the fluidtemperature Ta+200° C. (in fact, a temperature Tc of the thermallysensitive resistor 2+160° C.), and the thermally sensitive resistor 2 isheated to the fluid temperature Ta+40° C.

[0044] With the construction described above, when the forward flow 11generates in the main passage 4, the fluid flowing in the forwarddirection flows into the first sub-passage and the heating resistor 1 iscooled.

[0045] On the other hand, the forward flow 11 flows into the secondsub-passage in a less amount and develops a small cooling action uponthe thermally sensitive resistor 2. Also, since the thermally sensitiveresistor 2 is heated to be 40° C. higher than the fluid temperature inthe second sub-passage, the thermally sensitive resistor 2 is not heatedby the heat radiated from the heating resistor 1. Accordingly, theheating resistor 1 is held in the same state as when it is heated toTa+200° C.

[0046] Conversely, when the backward flow 12 generates in the mainpassage 8, the backward flow 12 acts upon the second sub-passage and thethermally sensitive resistor 2 is cooled. Although the thermallysensitive resistor 2 is heated in advance to be 40° C. higher than thefluid temperature in the second sub-passage, the temperature of thethermally sensitive resistor 2 lowers under cooling due to the backwardflow.

[0047] The backward flow 12 flows into the first sub-passage in a lessamount and develops a small cooling action upon the heating resistor 1.

[0048] In that case, the temperature of the thermally sensitive resistor2 lowers from the temperature to which it has been heated so far, andshows a value close to Ta. Also, the temperature of the heating resistor1 lowers and shows a value close to Ta+160° C. Therefore, the amount ofheat radiated to the fluid is decreased, and the output of the heatingresistor 1 is reduced. As a result, a plus error caused upon thegeneration of the backward flow is canceled as described later.

[0049] In the present invention, as described above, the thermallysensitive resistor 2 is heated in advance to be higher than the fluidtemperature in the sub-passage, and the heating resistor 1 and thethermally sensitive resistor 2 are arranged such that the cooling effectupon them differs from each other between when the forward flow 11 flowsin the main passage 4 and when the backward flow 12 flows in the mainpassage 4.

[0050] With those features, since the heating resistor 1 has a highdetection sensitivity for the forward flow 11 in the main passage 4 anda low detection sensitivity for the backward flow 12 in the main passage4, the later-described plus error caused upon the generation of thebackward flow is canceled and the flow rate in the forward direction canbe precisely detected even for a flow that may cause a pulsationaccompanying a backward flow, such as intake air of an automobileengine.

[0051] A measurement error caused by a pulsation in a prior-art flowrate measuring instrument will now be described with reference to FIG.2, taking as an example measurement of the flow rate of intake air inthe automobile engine.

[0052]FIG. 2 is a graph showing the relationship between an air pressurein an intake manifold (hereinafter referred to as an “in-manifoldpressure”), which is a pressure upstream of an engine cylinder, and anoutput of an air flow rate measuring instrument resulting when theengine revolution speed, i.e., the pulsation period, is held constant.

[0053] In FIG. 2, the in-manifold pressure changes depending on theopening degree of a throttle valve (throttle opening), which controlsthe flow rate of the intake air in interlock with an accelerator pedal.

[0054] The air flow rate measuring instrument is usually disposedupstream of the throttle valve. When the throttle opening is small, anintake pulsation is small due to the contraction effect of an intakepipe, and a measurement error caused by the pulsation does not occur inthe air flow rate measuring instrument. Therefore, an output of the airflow rate measuring instrument increases monotonously depending on thein-manifold pressure.

[0055] However, the intake pulsation increases with an increase of thethrottle opening. In a pulsation range where a minimum value of thepulsation amplitude approaches 0, the output of the air flow ratemeasuring instrument is reduced in spite of the true intake flow rateincreasing, thus resulting in a minus error.

[0056] Such a phenomenon is called a two-value phenomenon because theoutput of one air flow rate measuring instrument has two different flowrate values from each other.

[0057] The two-value phenomenon is primarily attributable to that therelationship of flow rate versus output of the heating resistor air flowrate measuring instrument is non-linear and the heating resistor airflow rate measuring instrument has a response delay. The techniquecalled a bypassing method is known as a measure for coping with thetwo-value phenomenon.

[0058] According to the bypassing method, a heating resistor is disposedin a bypass passage having a bent flow path formed therein so that themeasured value is shifted toward the plus side based on the bypassinginertia effect upon the generation of a pulsation, thereby canceling theminus error. That technique is known and therefore a detailed describedthereof is omitted here.

[0059] The present invention provides a technique capable of not onlymaintaining the above-mentioned bypassing effect, but also coping withthe two-value phenomenon in more satisfactory manner.

[0060] Furthermore, near a throttle fully-opened stroke in a specificrange of revolution speed, there occurs a pulsation in which a minimumvalue of the pulsation amplitude becomes below 0, i.e., a pulsationaccompanying a backward flow. At that time, the output of the air flowrate measuring instrument jumps up to a large extent, thus resulting ina large plus error.

[0061] Such a plus error caused by the backward flow will be describedwith reference to FIGS. 3 to 5.

[0062]FIG. 3 is a graph showing the relationship between a jumping-up(plus error), which is caused in a prior-art heating resistor flow ratemeasuring instrument upon the generation of a pulsation accompanying abackward flow near a throttle fully-opened stroke, and an output of theflow rate measuring instrument, the graph being expressed as a waveformon an assumption that there is no response delay.

[0063] In FIG. 3, the heating resistor cannot discriminate the directionof the fluid flow and hence detects the backward flow as the forwardflow in spite of that the actual backward flow should be detected as aminus value, as indicated by a dotted line. Accordingly, when thebackward flow occurs, the average value (integrated value) of an outputof the heating resistor includes a large positive error relative to thetrue value.

[0064]FIG. 4 is a graph showing an output of the so-called bypassingflow rate measuring instrument in which the heating resistor is lessexposed to the backward flow by providing a sub-passage having a bentportion, for example, with intent to reduce the plus error, shown inFIG. 3, caused in the prior-art heating resistor air flow rate measuringinstrument, the output being resulted when a pulsation accompanying abackward flow occurs, on an assumption that there is no response delayas in the graph of FIG. 3.

[0065] As seen from FIG. 4, optimization of the bypass passage canrealize a structure in which, even when the backward flow generates inthe main passage, the backward flow occurred in the bypass passage wherethe heating resistor is disposed can be held down very small. With thatstructure, it is possible to avoid the plus count of a minus value,which is caused because the heating resistor cannot detect whether thedirection of the fluid flow is forward or backward.

[0066] However, optimization of the bypass passage cannot completelycancel the plus error caused by the backward flow. The reason is thatwhile the true flow rate is given as the average value (integratedvalue) including the minus value resulting from the backward flow, thebypassing method provides the average value (integrated value) largerthan the true value because the backward flow is just cut off withoutincluding the minus value in the average value (integrated value).

[0067] In other words, unless the output resulting from the forward flowis deduced from a total flow rate depending on the backward flow asindicated by a dotted line, the true flow rate and the output averagevalue do not match with each other.

[0068]FIG. 5 is a graph showing an actual output of the heating resistorflow rate measuring instrument as compared with the graph of FIG. 4 inwhich there is no response delay. The actual output of the heatingresistor flow rate measuring instrument has a less-sharpened waveform,i.e., a waveform having a gentler slope than that of the true output,due to the response delay in the outputting.

[0069] In the actual heating resistor flow rate measuring instrumentcausing the response delay, as shown in FIG. 5, the plus error caused bythe backward flow is reduced because of the two-value phenomenondescribed above as the known phenomenon, but it is not completelycanceled.

[0070] The present invention is able to further compensate for themeasurement error caused by the backward flow, which is not completelycanceled by the bypassing method, while maintaining the bypassingeffect.

[0071]FIG. 6 is a graph showing the relationship between a jumping-up(plus error), which is caused in the heating resistor flow ratemeasuring instrument of the present invention upon the generation of apulsation accompanying the backward flow near a throttle fully-openedstroke, and an output of the flow rate measuring instrument.

[0072] In the present invention, the heating resistor 1 is disposed inthe bypass passage described above so as to suppress the two-valuephenomenon based on the bypassing effect and to reduce the plus errorcaused by the backward flow. Further, in the present invention, asdescribed above with reference to FIG. 1, the thermally sensitiveresistor 2 is disposed to be more easily exposed to the backward flowthan to the forward flow, and it is heated in advance to a level higherthan the fluid temperature in the sub-passage by the predeterminedtemperature. With those features, when the backward flow 12 generates inthe main passage 4, the temperature to which the heating resistor 1 isheated becomes lower, whereby the plus error can be further reducedwhich is caused in the heating resistor air flow rate measuringinstrument upon the generation of a pulsation accompanying the backwardflow.

[0073] A dotted line in FIG. 6 represents the output of the heatingresistor air flow rate measuring instrument resulting when the influenceof the backward flow is reduced by the optimization of the bypassingeffect described above, while a solid line represents the output of theheating resistor air flow rate measuring instrument of the presentinvention.

[0074] With the present invention, for the fluid flowing only as theforward flow without including the backward flow, the flow ratemeasuring instrument produces the same output as that obtained with thebypassing method. Upon the generation of a pulsation accompanying thebackward flow, however, the thermally sensitive resistor 2 is cooled bythe backward flow, whereupon the temperature to which the heatingresistor 1 is heated becomes lower and the amount of heat radiated fromthe heating resistor 1 is reduced. Accordingly, the output of theheating resistor air flow rate measuring instrument is reduced.

[0075] In practice, since the temperature of the thermally sensitiveresistor 2 is maintained in a lowered state due to the response delay inthe states of not only the backward flow, but also the forward flow, theinstrument produces the output shifting toward the minus direction as awhole.

[0076] Thus, the plus error caused by the backward flow is reduced andthe flow rate can be precisely detected even for a flow that may cause apulsation accompanying the backward flow, such as intake air of anautomobile engine.

[0077] A heating resistor air flow rate measuring instrument as apractical embodiment of the present invention will be described belowwith reference to FIGS. 7 to 13.

[0078]FIG. 7 is a sectional view showing a state in which a heatingresistor flow rate measuring instrument according to a first embodimentof the present invention is mounted as a module in the main passage 4.FIG. 8 is a sectional view taken along the line A-A in FIG. 7, and FIG.9 is a sectional view taken along the line B-B in FIG. 7.

[0079] Referring to FIGS. 7 to 9, the heating resistor 1 and thethermally sensitive resistor 2 are disposed respectively in a firstbypass 302 and a sub-passage (for arrangement of the thermally sensitiveresistor) 306 of a bypass 3 while being fixed to corresponding terminals14, and they are electrically connected to an electronic circuit 5 viametal wires 15.

[0080] A housing 6 for protectively containing the electronic circuit 5is a plastic-molded part formed by insert molding with the terminals 14and connector terminals 9 incorporated therein as metal terminals.

[0081] The housing 6 comprises the bypass 3 constituting a bent flowpath in which the heating resistor 1 is disposed, a passage portionpartly constituting a sub-passage 4 in which the thermally sensitiveresistor 2 is disposed, a case portion forming a frame in which theelectronic circuit 5 is mounted in a protective way, a connector portionin which the connector terminals 9 are disposed for electricalconnection to an external device, and a flange portion used for fixingthe flow rate measuring instrument to a member 16 that constitutes amain passage 4 described later. These components are formed integrallywith each other.

[0082] The housing 6 and the electronic circuit 5 are fixedly bonded toa metal base 7, and the bypass 3 and the sub-passage 306 are completedby joining of a bypass cover 13 and a circuit cover 10 in respectiveplaces such that the electronic circuit 5 is protected at itssurroundings. Thus, the flow rate measuring instrument is constructed asa module containing the circuit, the sensors, the sub-passage, theconnector, etc. in an integral structure.

[0083] The main passage 4 is a flow path through which a fluid to bemeasured flows, and it corresponds to an intake pipe extending from anair cleaner to a position upstream of an engine cylinder, for example,when the present invention is applied to an automobile engine.

[0084] In a heating resistor air flow rate measuring instrument for usein an automobile, the member 16 constituting the main passage 4 isconstituted as a body dedicated to the heating resistor air flow ratemeasuring instrument and connected to midway the intake pipe, or it isconstituted by employing an air cleaner, a duct, a throttle body or thelike in common.

[0085] An insertion hole 17 is formed in a wall of the main passageconstituting member 16, and a measuring unit containing both the heatingresistor 1 and the thermally sensitive resistor 2 mounted therein isinserted through the insertion hole 17 so as to position in the mainpassage 4. By fixing the housing 6 to the main passage constitutingmember 16, the instrument is set in a state capable of measuring theflow rate of air flowing through the main passage.

[0086] The bypass 3 is formed as a roundabout path made up of a bypassinlet 301 opened in a plane substantially perpendicular to a center axisof the main passage 4, a first bypass 302 extending substantiallyparallel to the center axis of the main passage 4, a roundabout portion303 for reversing 1800 the flow direction at a downstream end of thefirst bypass 302, a second bypass 304 extending substantially parallelto the first bypass in the opposite direction, and a bypass outlet 305opened at a downstream end of the second bypass 304 in a planesubstantially perpendicular to the center axis of the main passage 4.

[0087] The heating resistor 1 is disposed, as described above, in thefirst bypass 302 of the roundabout bypass 3. Thus, a structure isobtained in which, when the backward flow 12 generates in the mainpassage 4, the backward flow is hard to flow in up to the location wherethe heating resistor 1 is disposed.

[0088] The sub-passage 306 is formed as a straight tubular path having asub-passage inlet 401 opened in a plane substantially perpendicular tothe center axis of the main passage 4, and a sub-passage outlet 402formed downstream of the sub-passage inlet.

[0089] A projection-shaped flow restrictor 403 is formed in the tubularpath of the sub-passage 306, and the thermally sensitive resistor 2 isdisposed in a position shielded by the flow restrictor 403 when viewedfrom the upstream side.

[0090] Accordingly, when the forward flow 11 flows in the main passage4, the presence of the flow restrictor 403 forms a region in which thefluid flow turns to a separated flow at a location where the thermallysensitive resistor 2 is disposed. Hence, the separated flow has a verylow flow speed as compared with that of a flow reaching the locationwhere the thermally sensitive resistor 2 is disposed when the backwardflow 12 flows in the main passage 4.

[0091] The electronic circuit 5 is constituted as containing a circuitshown in FIG. 1 and has a bridge circuit including the heating resistor1 (Rh) and the thermally sensitive resistor 2 (Rc). More specifically,one end of the heating resistor 1 is connected to the other end of theheating resistor 1 through resistors R1, R2, the thermally sensitiveresistor 2, and a resistor R3.

[0092] Further, the junction between the resistors R1 and R2 isgrounded, and the junction between the thermally sensitive resistor 2and the resistor R2 is connected to one input terminal of an operationalamplifier OP1. An output terminal of the operational amplifier OP1 isconnected to the base of a transistor Tr, and the emitter of thetransistor Tr is connected to the junction between the resistor R3 andthe heating resistor 1.

[0093] In addition, the junction between the thermally sensitiveresistor 2 and the resistor R1 is connected to the other input terminalof the operational amplifier OP1.

[0094] The junction between the thermally sensitive resistor 2 and theresistor R1 is also connected to one input terminal of an operationalamplifier OP2, and further connected to an output terminal of theoperational amplifier OP2 through a resistor R4.

[0095] The other input terminal of the operational amplifier OP2 isconnected to a reference voltage source Vref through a resistor R5 andis also grounded through a resistor R6.

[0096] After balancing the bridge circuit including the heating resistor1 and the thermally sensitive resistor 2, electric currents Ih and Icflowing respectively through the heating resistor 1 and the thermallysensitive resistor 2 are adjusted so that the heating resistor 1 issufficiently heated (for example, to the fluid temperature+200° C.,i.e., the temperature of the thermally sensitive resistor+160° C.) andthe thermally sensitive resistor 2 is slightly heated (for example, tothe fluid temperature+40° C.).

[0097] With the construction described above, the flow rate can alwaysbe precisely measured even for a flow that may change from a steady flowto a pulsating flow and further to a pulsating flow accompanying abackward flow, such as intake air of an automobile engine.

[0098] Stated another way, when a pulsating flow generates and apulsation increases to such an extent that a minimum flow speed becomesclose to 0, the minus error called the two-value phenomenon occurs asdescribed above. By arranging the heating resistor 1 in the roundaboutbypass 3, however, the minus error can be suppressed and canceled withthe bypassing inertia effect mentioned above.

[0099] Further, when the pulsation amplitude increases and generates apulsating flow accompanying a backward flow, the structure of theroundabout bypass 3 serves to suppress the backward flow from flowing inup to the location where the heating resistor 1 is disposed.

[0100] Those advantages are also obtained with the prior art, but areduction of the plus error caused by the backward flow is not yetsufficient, as described above.

[0101] With the first embodiment of the present invention, since thethermally sensitive resistor 2 slightly heated is disposed in thesub-passage 306 such that the thermally sensitive resistor 2 is harderto be exposed to the forward flow and is more easily exposed to thebackward flow, the thermally sensitive resistor 2 is cooled by the fluidupon the generation of the backward flow and the temperature of theheating resistor 1, which is controlled to be held at a level higherthan that of the thermally sensitive resistor 2 by the predeterminedtemperature, is reduced.

[0102] Accordingly, the amount of heat radiated from the heatingresistor 1 to the fluid is also reduced and so is an electric currentsupplied for the heating. In other words, the measured value of the flowrate is shifted toward the minus side upon detection of the backwardflow, whereby the plus error caused by the backward flow is canceled andthe measurement can be made at higher accuracy.

[0103] In practice, the thermally sensitive resistor 2 is also cooled bythe forward flow depending on the flow rate, but the measurementaccuracy in the state of the forward flow can be maintained by measuringthe relationship between the flow rate and the output in the state ofthe forward flow beforehand and obtaining an output characteristic ofthe heating resistor air flow rate measuring instrument based on themeasured relationship.

[0104] Thus, the highly precise measurement can be achieved not only inthe state of the forward flow, but also when a pulsation accompanyingthe backward flow occurs.

[0105]FIG. 10 is a schematic sectional view of a sub-passage 306 in aheating resistor flow rate measuring instrument according to a secondembodiment of the present invention. The other construction than thesub-passage is the same as that of the above-described first embodiment,and hence illustration and a detailed description thereof are omittedhere.

[0106] In the second embodiment of the present invention, theconfiguration of the sub-passage 306 is somewhat modified from that inthe first embodiment.

[0107] Note that, similarly to FIG. 9, FIG. 10 is a sectional view takenalong the line B-B in FIG. 7.

[0108] In the second embodiment, as shown in FIG. 10, a partition 404extending in the lengthwise direction of the sub-passage 306 is formedin the sub-passage 306 for the purpose of further reducing the flowspeed of the fluid in the state of the forward flow at the locationwhere the thermally sensitive resistor 2 is disposed.

[0109] Additionally, the partition 404 has a taper 405 formed so as todefine a tubular path gradually widening toward the downstream side ofthe sub-passage 306. The presence of the taper 405 contributes toincreasing the amount of the backward flow flowing toward the side nearthe thermally sensitive resistor 2.

[0110] The sub-passage 306 thus constructed can provide similaradvantages to those obtained with the first embodiment. In addition,since the second embodiment produces a larger difference in temperatureof the thermally sensitive resistor 2 between the state of the forwardflow and the state of the backward flow than that in the firstembodiment, a larger minus shift is obtained in the state of thebackward flow and the plus error caused by the backward flow can befurther reduced.

[0111]FIG. 11 is a schematic sectional view of a sub-passage 306 in aheating resistor flow rate measuring instrument according to a thirdembodiment of the present invention. The other construction than thesub-passage is the same as that of the above-described first embodiment,and hence illustration and a detailed description thereof are omittedhere.

[0112] In the third embodiment of the present invention, as in thesecond embodiment, the configuration of the sub-passage 306 is somewhatmodified from that in the first embodiment.

[0113] As shown in FIG. 11, the sub-passage 306 is formed such that itprovides a simple tubular path 407 in the upstream side, abruptlynarrows at the downstream end of the tubular path 407, and then providesa tubular path 406 gradually widening toward the downstream side. Thethermally sensitive resistor 2 is disposed at the downstream end of thetubular path 407 to lie on an extension of the center axis of thegradually widening tubular path 406.

[0114] With the sub-passage 306 according to the third embodiment, inthe state of the forward flow, the resistance against passage of thefluid increases and the flow rate of the incoming fluid reducescorrespondingly. In the state of the backward flow, the backward flowflows as a jet into the simple tubular path 406 from the graduallywidening tubular path 407, and therefore the effect of cooling thethermally sensitive resistor 2 is increased.

[0115] The third embodiment can also provide similar advantages to thoseobtained with the first embodiment.

[0116] Next, a heating resistor flow rate measuring instrument accordingto a fourth embodiment of the present invention will be described withreference to FIGS. 12 and 13. In this fourth embodiment, the thermallysensitive resistor 2 is heated in a different manner from that in thefirst embodiment. FIG. 12 is a cross-sectional view of an instrumentmodule with the main passage 4 omitted, and FIG. 13 is an externalappearance view, looking from the upstream side, of the instrumentmodule. Note that FIG. 12 corresponds to a section taken along the lineD-D in FIG. 13.

[0117] An overall construction of the fourth embodiment is the same asthat of the first embodiment, and therefore the following description ismade of only different points from the first embodiment.

[0118] The biggest difference between the first embodiment and thefourth embodiment is as follows. In the first embodiment, the thermallysensitive resistor 2 is of the self-heating type that an electriccurrent is applied to the thermally sensitive resistor 2 to heat it. Inthe fourth embodiment, however, the thermally sensitive resistor 2 isnot of the self-heating type, and a separate heater 20 is disposedupstream of the thermally sensitive resistor 2 so that the thermallysensitive resistor 2 is heated by the heater 20.

[0119] More specifically, in the fourth embodiment, a heat flowgenerated under heating by the heater 20 heats the thermally sensitiveresistor 2 in the state of the forward flow. On the other hand, in thestate of the backward flow, the thermally sensitive resistor 2 is notaffected by the temperature of the heater 20 and is cooled so as toapproach the fluid temperature. Therefore, the temperature of thethermally sensitive resistor 2 differs between the state of the forwardflow and the state of the backward flow.

[0120] In the fourth embodiment, the sub-passage 306 can be formed as asimple tubular path, and a flow restrictor or the like is not required.Further, the temperature to which the thermally sensitive resistor 2 isheated in the state of the forward flow can be adjusted depending on thespacing between the heater 20 and the thermally sensitive resistor 2 andon an extent by which they overlap with each other.

[0121] The fourth embodiment can also provide similar advantages tothose obtained with the first embodiment.

[0122] While the heating resistor 1 and the thermally sensitive resistor2 are disposed in the sub-passage 3 in the embodiments described above,the locations where the heating resistor 1 and the thermally sensitiveresistor 2 are disposed are not limited to positions in the sub-passage.

[0123] Specifically, the heating resistor 1 may be disposed at a firstlocation within the main passage 4 where the heating resistor 1 isexposed to the fluid flowing in one direction within the main passage 4in a larger amount than the fluid flowing in a direction opposite to theone direction, while the thermally sensitive resistor 2 may be disposedat a second location within the main passage 4 where the thermallysensitive resistor 2 is exposed to the fluid flowing in the oppositedirection within the main passage 4 in a larger amount than the fluidflowing in the one direction within the main passage 4.

[0124] Also, the first sub-passage may have a first opening that facessubstantially perpendicular to the fluid flowing in the forwarddirection (one direction), and a second opening that faces substantiallyperpendicular to the fluid flowing in the opposite direction and has asmaller opening area than the first opening. The second sub-passage mayhave a third opening that faces substantially perpendicular to the fluidflowing in the forward direction, and a fourth opening that facessubstantially perpendicular to the fluid flowing in the oppositedirection and has a larger opening area than the third opening. Then,the heating resistor 1 may be disposed in the first sub-passage and thethermally sensitive resistor 2 may be disposed in the secondsub-passage.

[0125] Another modification may be constructed as follows. A firstheating resistor radiating a larger amount of heat in the state of theforward flow than in the state of the backward flow and a second heatingresistor radiating a larger amount of heat in the state of the backwardflow than in the state of the forward flow are both disposed in the mainpassage 4, and a bridge circuit including the first heating resistor andthe second heating resistor is formed. Then, an applied electric currentis controlled so that the first heating resistor is held at atemperature higher than the second heating resistor by a certaintemperature. The flow rate of the fluid passing through the main passage4 is measured based on the amount of heat radiated from the firstheating resistor.

INDUSTRIAL APPLICABILITY

[0126] According to the present invention, in the heating resistor flowrate measuring instrument, when the fluid flows in the forwarddirection, the heating resistor 1 outputs a value depending on the flowrate of the fluid. When the fluid flows in the opposite direction, thethermally sensitive resistor 2 is cooled to a temperature lower thanthat resulting when the fluid flows in the forward direction, and anelectric current flowing through the heating resistor 1 is decreased. Asa result, even when a pulsation occurs in the fluid, a precise flow ratecan be measured because not only a plus effect caused by the backwardflow is eliminated, but also a value corresponding to the backward flowis subtracted.

[0127] It is hence possible to precisely detect the flow rate in theforward direction even for a fluid that may cause a pulsationaccompanying a backward flow, such as intake air of an automobileengine.

[0128] Particularly, in measurement of the flow rate of intake air of anautomobile engine, a large plus error resulting when a pulsationaccompanying a backward flow occurs near a throttle fully-opened strokein a specific range of revolution speed can be eliminated and fuelcontrol precisely responsive to operation conditions can be achieved.

1. A heating resistor flow rate measuring instrument comprising aheating resistor (1) and a thermally sensitive resistor (2) bothdisposed in a main passage (4), and measuring the flow rate of a fluidpassing through said main passage (4), said instrument including: afirst location exposed to the fluid flowing in one direction within saidmain passage (4) in a larger amount than the fluid flowing in adirection opposite to the one direction; and a second location exposedto the fluid flowing in the opposite direction within said main passage(4) in a larger amount than the fluid flowing in the one direction,wherein said heating resistor (1) is disposed at said first location,said thermally sensitive resistor (2) is disposed at said secondlocation, and the flow rate of the fluid passing through said mainpassage (4) is measured based on amounts of heat radiated from saidheating resistor (1) and said thermally sensitive resistor (2).
 2. Aheating resistor flow rate measuring instrument according to claim 1,wherein said heating resistor (1) is heated to be higher than a fluidtemperature in said main passage (4) by a first predeterminedtemperature, and said thermally sensitive resistor (2) is heated to behigher than the fluid temperature in said main passage (4) by a secondpredetermined temperature.
 3. A heating resistor flow rate measuringinstrument according to claim 1 or 2, wherein said first location isprovided by a first sub-passage (302) having a first opening that facessubstantially perpendicular to the fluid flowing in the one direction,and a second opening that faces substantially parallel to the fluidflowing in the opposite direction, and said second location is providedby a second sub-passage having a third opening that faces substantiallyperpendicular to the fluid flowing in the opposite direction, and athird opening that faces substantially parallel to the fluid flowing inthe one direction.
 4. A heating resistor flow rate measuring instrumentaccording to claim 1 or 2, wherein said first location is provided by afirst sub-passage (302) having a first opening that faces substantiallyperpendicular to the fluid flowing in the one direction, and a secondopening that faces substantially parallel to the fluid flowing in theopposite direction, and a wall portion (403) having a surfacesubstantially perpendicular to the lengthwise direction of said mainpassage is formed in the one-direction side of said second location. 5.A heating resistor flow rate measuring instrument according to claim 1or 2, wherein said first location is provided by a first sub-passagehaving a first opening that faces substantially perpendicular to thefluid flowing in the one direction, and a second opening that facessubstantially perpendicular to the fluid flowing in the oppositedirection and has a smaller opening area than said first opening, andsaid second location is provided by a second sub-passage having a thirdopening that faces substantially perpendicular to the fluid flowing inthe one direction, and a fourth opening that faces substantiallyperpendicular to the fluid flowing in the opposite direction and has alarger opening area than said third opening.
 6. A heating resistor flowrate measuring instrument according to any one of claims 1, 2, 3, 4 and5, wherein said thermally sensitive resistor is heated to a temperature20° C.-40° C. higher than the fluid temperature in said main passage. 7.A heating resistor flow rate measuring instrument for measuring the flowrate of a fluid passing through a passage (4), said instrumentcomprising: a first heating resistor radiating a larger amount of heatto the fluid flowing in one direction within said passage (4) than tothe fluid flowing in a direction opposite to the one direction; and asecond heating resistor radiating a larger amount of heat to the fluidflowing in the opposite direction than to the fluid flowing in the onedirection; wherein a bridge circuit including said first heatingresistor and said second heating resistor is formed, said first heatingresistor is heated to be higher than said second heating resistor by acertain temperature, and the flow rate of the fluid passing through saidpassage (4) is measured based on the amount of heat radiated from saidfirst heating resistor.
 8. A heating resistor flow rate measuringinstrument comprising a heating resistor (1) and a thermally sensitiveresistor (2) both disposed in a main passage (4), and measuring the flowrate of a fluid passing through said main passage (4), wherein: saidinstrument includes a thermally-sensitive-resistor arrangement locationin which said thermally sensitive resistor (2) is disposed and radiatesa larger amount of heat when the fluid flows in a direction opposite toone direction within said main passage (4) than when the fluid flows inthe one direction; said heating resistor (1) is heated to be higher thana fluid temperature in said passage (4) by a first predeterminedtemperature; said thermally sensitive resistor (2) is heated to behigher than the fluid temperature in said passage (4) by a secondpredetermined temperature; and the flow rate of the fluid passingthrough said passage (4) is measured based on amounts of heat radiatedfrom said heating resistor (1) and said thermally sensitive resistor(2).
 9. A heating resistor flow rate measuring instrument according toclaim 8, wherein said heating resistor (1) is disposed at a location inwhich said heating resistor (1) radiates a larger amount of heat whenthe fluid flows in the one direction within said main passage (4) thanwhen the fluid flows in the opposite direction.